1. Field of the Invention
The present invention relates to an alignment strategy optimization method for finding an optimal alignment strategy to process substrates in a lithographic manufacturing process.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a lithographic patterning device, which is alternatively referred to as a “mask” or “reticle,” may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g., comprising part of, one or several dies) on a substrate (e.g., a silicon wafer) that has a layer of radiation-sensitive material (i.e., resist).
In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, while in so-called scanners, each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
During the manufacturing process, a number of patterned layers may be created on a substrate. In order to create an operating device or to provide optimal performance, it may be desirable or even necessary for the patterns of layers positioned on top of each other to be well aligned with respect to each other. Such a condition may be accomplished by accurately positioning the substrate with respect to the mask and the projection beam. In the first place, it may be desirable or necessary for the substrate to be in the focal plane of the patterned beam, in order to obtain a sharp image of the patterning structure (a process also known as “focus and leveling”). The direction associated with this distance is called the Z-direction.
Secondly, it may be desirable or necessary to accurately set the position of the substrate in the directions perpendicular to the Z-direction, i.e. the X- and Y-direction, in order to position the different layers correctly on top of each other (a process also known as “aligning”). Accurate aligning is generally done by accurately determining the position of the substrate relative to a substrate table which holds the substrate and determining the position of the substrate table with respect to the mask and projection beam. Alignment may be done using an alignment system, as described, for instance in U.S. Pat. No. 6,297,876, which is incorporated herein by reference.
Alignment is performed using the alignment system which is arranged to find the position of alignment markers on the substrate. The performance of the alignment system is one of the elements of a lithographic system that influences the overlay accuracy to a large extent. During alignment, multiple marks on the substrate are measured to obtain a coordinate system. Some advanced IC processes alter the geometry of the alignment marks, which may compromise the coordinate system. ASML's ATHENA™ Phase-Grating Alignment system offers extensive operational flexibility to cope with most advanced IC processes, because of its dual-wavelength operation and its simultaneous detection of up to the seventh diffraction order. A more extensive overview of the ATHENA alignment sensor system and its basic operation is provide in F. Bornebroek et al., “Overlay Performance in Advanced Processes”, Proc. SPIE Microlithography, Vol. 4000 (2000) pp. 520–531, which is herein incorporated by reference. The ATHENA system offers great flexibility in applying an optimal alignment strategy, see, e.g., P. Hinnen et al., “Advances in Process Overlay”, Proc. SPIE Microlithography, Vol. 4344 (2001) pp 114–125.
A diffraction pattern (e.g. as generated by an alignment beam projected to an alignment mark) may comprise a number of diffraction orders, and some number (for instance, seven) of the diffraction orders may be measured. Each diffraction order comprises positional information about the alignment mark. In many cases, a position of the alignment mark can be determined based on the determined position of a single diffraction order, but more accurate results may be obtained when more diffraction orders are taken into account.
The position of the substrate may be expressed by wafer model parameters such as a translation T, a rotation R, and an magnification M. The translation may be in the X-direction Tx and/or in the Y-direction Ty. The rotation may be a rotation of the x-axis about the z-axis Rzx and/or a rotation of the y-axis about the z-axis Rzy. The magnification may in the X, Mx, and/or in the Y-direction. The wafer model parameters (Tx, Ty, Rzx, Rzy, Mx, My) can be used to compute the position, magnification and/or orientation of a substrate based on the measured positions of the diffraction orders. The wafer model parameters can be used to find the optimal alignment strategy.
Alignment strategies may consist of choice of mark type and location as well as the choice of the diffraction order and wavelength to be used. The appropriate selection procedure is chosen depending on the environment (i.e. research or production). For any alignment strategy, it is possible to automate the wavelength selection during the lithographic process. For every mark, the optimal wavelength is selected based on the signal strength of each diffraction order.
Choosing the optimal strategy is important in obtaining optimal overlay. Different procedures for selecting alignment strategy have been developed to comply with different applications. Depending on the application, either a comprehensive technique to determine the ultimate strategy or a fast and adequate strategy—optimization technique is recommended. These procedures for selecting an alignment strategy calculate an ‘overlay indicator’ for every possible alignment strategy. The strategy with the lowest indicator is recommended, since it corresponds to a minimum process-induced overlay variance over a batch of substrates. Overlay indicators can be insensitive to processing effects that are constant over the batch, since process corrections are used to correct for these effects during production.
To select an alignment strategy, multiple alignment mark types are measured. Each measurement gives for example 14 positions from seven diffraction orders at two wavelengths. Various procedures for selecting an alignment strategy are known, such as, for example, the Overlay Metrology Tool Feedback (OVFB) procedure. For this procedure, multiple processed marks on multiple substrates are measured in a process free coordinate system. The principle of the overlay metrology tool feedback analysis is explained with reference to FIGS. 1A, 1B, 1C, 1D. In this example, only the Y-direction is discussed for reasons of simplicity.
FIG. 1A shows a substrate 1 comprising four alignment marks 2, 3, 4, 5 of a first mark type and four alignment marks 6, 7, 8, 9 of a second mark type. During production, the alignment marks 2, 3, 4, 5 are used for alignment when exposing the substrate 1. Vectors in FIG. 1A indicate a relative position of the alignment marks 2, 3, 4, 5 with respect to an active (i.e. used) grid. In addition, the alignment marks 6, 7, 8, 9 of the second mark-type are measured as well. The substrate 1 is developed and overlay is measured on an offline metrology tool.
FIG. 1B shows the substrate 1 and four overlay targets 10, 11, 12, 13 together with vectors indicating the measured overlay error. A model is applied to the measured overlay values (i.e. the length of the vectors) in order to determine the (in this example only one) wafer model parameter(s) (Ty). The (se) wafer model parameter(s) are used to calculate alignment errors of the alignment marks 2, 3, 4, 5 used for exposure and of the alternative alignment marks 6, 7, 8, 9, (see vectors in FIG. 1C).
Based on the alignment errors in FIG. 1C, a mark-type is chosen. This choice can be made in different ways. For example, as will be explained below, the value of an “Overlay Performance Indicator” (OPI) may be used. Alternatively, the alignment errors in FIG. 1C may be used to calculate the possible overlay when switching alignment marks, and the mark-type which causes the lowest overlay will then be chosen. In this example, a switch could be made from using alignment marks 2, 3, 4, 5 of the first mark type to the alignment marks 6, 7, 8, 9 of the second mark type. In FIG. 1D, the overlay errors in that case are indicated by vectors, which apparently are smaller than those in FIG. 1C.
The calculation of OPI will now be discussed. The first action is to determine the wafer model parameters for a certain alignment strategy. The batch averages are subtracted, because process correction can compensate for these. The OPI is defined as the mean plus 3 times the standard deviation of the maximum expected overlay error of each substrate. The maximum expected overlay error Max_err for a 4 parameter wafer model is given by:Max—err=√{square root over (Tx2+Ty2)}+wafer—radius·√{square root over (R2+M2)},  (1)with R=(Rzx+Rzy)/2, M=(Mx+My)/2where Tx is a translation in the x-direction, Ty a translation in the y-direction, Rzx a rotation of the x-axis about the z-axis, Rzy a rotation of the y-axis about the z-axis, Mx an magnification in the direction of the x-axis, My a magnification in the direction of the y-axis, wafer_radius the radius of a substrate.Now, the OPI is given by:OPI=<Max—err>+3·σ(Max—err)  (2)where σ is a standard deviation over all substrates.
For a 6 parameter model, it is not possible to calculate the OPI analytically. The 6 parameter OPI value has to be calculated numerically. Once a value for the OPI for all strategies is calculated, the alignment strategy with the lowest OPI is selected since it is believed that that particular strategy results in minimal overlay.
Nowadays, overlay indicator values are calculated on single batches. Overlay indicators can be based on alignment or alignment plus overlay data. The confidence level of the indicators based on both data sources is significantly higher than the indicator based on alignment data only. Based on the values of the indicators for different alignment strategies, a decision is made on which strategy to use to expose future batches.
Alignment strategy optimization is done by calculating the values of overlay indicators on single batches that need to be fully measured on an offline metrology tool in order to get high confidence values. During regular operations, however, only a few substrates out of a batch are measured, therefore the high confidence level indicators can not be used and the alternative indicator is used which has a very low confidence level. Measuring of extra substrates on the offline metrology tool costs extra effort and time.